The present invention relates to an imaging apparatus utilizing a laser beam or laser beams, and more particularly, to an imaging device that scans an objective surface, on which an image is formed, with the scanning beam(s) modulated in accordance with raster data representing an image to be formed, and simultaneously controls a table on which the objective surface is located to move.
Conventionally, imaging apparatuses utilizing a laser beam are widely used for forming minute patterns on objective surfaces. As a typical example of application of such an imaging apparatus, formation of circuit patterns for a printed circuit board in accordance with a photolithograph method is widely known. A photo resist layer is formed on a base board, on which, the circuit pattern is to be formed. The circuit pattern is designed/edited at the CAD/CAM stations, and then transmitted to an imaging apparatus as vector data. At the imaging device, the vector data of the circuit pattern is converted into raster data. Based on the raster data, and in accordance with a clock pulse having a predetermined frequency, the circuit pattern is formed on the photo resist layer on the base board.
In order to form the printed circuit board at high accuracy, formation of the circuit pattern is performed such that the size and position of the image are adjusted precisely. Assigning of an actual size to the circuit pattern (which is called as a xe2x80x9cscalingxe2x80x9d) is performed at the CAM station, and the adjustment of the position with respect to the objective surface is performed at the imaging apparatus by positioning the objective surface accurately with respect to the scanning beam when the image is formed. However, it is unavoidable that a base board have individual differences depending on the manufacturing conditions and ambient conditions, which result in differences of board size among the base boards. If the size error exceeds an allowable range, the scaling of the circuit pattern should be compensated so as to meet the base board.
In Japanese Patent Provisional Publication HEI 9-323180, an imaging device which compensates for the scaling in order to form an image on a base board which is evenly contracted or expanded in a main or an auxiliary scanning direction is described. In this publication, the size error in a main or an auxiliary scanning direction is compensated using an appropriate clock signal which is generated by gradually shifting a phase of a reference clock pulse. However, if the size of the base board changes unevenly, e.g., the size of the board gradually changes in the main or auxiliary scanning direction, the above-described scaling compensation may not work sufficiently.
It is therefore an object of the invention to provide an improved laser imaging apparatus which is capable of compensating for the size of an image, e.g., a circuit pattern, in correspondence with a relatively complicated contraction/expansion of an objective surface, on which the image is to be formed.
According to an aspect of the invention, there is provided a laser imaging apparatus, which is provided with a scanning unit that emits at least one scanning laser beam, which scans on a surface to be scanned in a main scanning direction to form a scanning section, the at least one scanning laser beam being modulated in accordance with image data, a moving system that moves the surface to be scanned in an auxiliary scanning direction which is parallel to the surface to be scanned and different from the main scanning direction, a rotating system that rotates the surface to be scanned about a predetermined rotation axis, a detecting system that detects a plurality of position marks formed on the surface to be scanned, a calculating system that calculates an inclination of the scanning section with respect the main scanning direction, and a scaling factor of the scanning section with respect to a predetermined length in the main scanning direction based on a shape of the surface to be scanned, the shape being determined in accordance with the positions of the plurality of position marks, and an adjusting system that adjusts the rotational position of the surface to be scanned in accordance with the inclination, and a start position of the scanning section on the surface to be scanned in accordance with the scaling factor.
Since the rotational position and start position of each scanning section is determined and adjusted, even if the shape of the surface to be scanned is changed gradually, the image can be formed on the surface precisely.
Optionally, the plurality of position marks include four position marks defining a quadrilateral, and the calculating system determines the inclination of the scanning section located at the sides, which are aligned along the auxiliary scanning direction (i.e., extends substantially in the main scanning direction), of the quadrilateral based on the coordinates of the four position marks, the calculating system determining the inclination of the scanning section located between the sides of the quadrilateral by applying a linear approximation to the inclinations of the scanning section located at the sides.
In particular, the four position marks include first and second marks P1, P2 aligned in the main scanning direction, and third and fourth marks P3 and P4 aligned in the main scanning direction, the first and second marks P1 and P2 being spaced from the third and fourth marks P3 and P4 in the auxiliary scanning direction, two-dimensional coordinates of the marks P1, P2, P3 and P4 as detected being defined as (PX1, PY1), (PX2, PY2), (PX3, PY3) and (PX4, PY4), the inclinations of the scanning sections located at the sides of the quadrilateral being expressed as:
S1=(PX2xe2x88x92PX1)/(PY2xe2x88x92PY1); and
SN=(PX4xe2x88x92PX3)/(PY4xe2x88x92PY3).
In this case, the surface to be scanned is rotated for each scanning section to compensate for the inclination with respect to the main scanning direction, an imaging start position of each scanning section being compensated in accordance with a distance of the scanning section from the axis of rotation and an angle of rotation.
Still optionally, the plurality of position marks may include four position marks defining a quadrilateral, wherein the calculating system determines the scaling factors of the scanning sections located at the sides, which are aligned along the auxiliary scanning direction, of the quadrilateral based on the coordinates of the four position marks, the calculating system determining the scaling factor of the scanning section located between the sides of the quadrilateral by applying a linear approximation to the scaling factors of the scanning section located at the sides.
In particular, the four position marks include first and second marks P1, P2 aligned in the main scanning direction, and third and fourth marks P3 and P4 aligned in the main scanning direction, the first and second marks P1 and P2 being spaced from the third and fourth marks P3 and P4 in the auxiliary scanning direction, two-dimensional coordinates of the marks P1, P2, P3 and P4 as detected being defined as (PX1, PY1), (PX2, PY2), (PX3, PY3) and (PX4, PY4), two-dimensional coordinates of positions Q1, Q2, Q3 and Q4, which represent designed positions of the four marks P2, P2, P3 and P4, being (QX1, QY1), (QX2, QY2), (QX3, QY3) and (QX4, QY4), the scaling factors T1 and TN of the scanning sections located at the sides of the quadrilateral being expressed as:
T1=(PY2xe2x88x92PY1)/(QY2xe2x88x92QY1); and
TN=(PY4xe2x88x92PY3)/(QY4xe2x88x92QY3).
Further optionally, the laser imaging apparatus may include a first table that mounts a member, a surface of which being the surface to be scanned, wherein the rotating system includes a second table that mounts the first table, the first table rotating as the second table rotates, and wherein the moving system includes a third table that mounts the second table, the first and second table moving in the auxiliary scanning direction as the third table moves in the auxiliary scanning direction.
Yet optionally, the auxiliary scanning direction is perpendicular to the main scanning direction.
Further optionally, the detecting system may includes at least one camera system fixed with respect to the imaging apparatus, the camera capturing an image of the plurality of position marks, and an image processing system that processes the image captured by the at least one camera and determines two-dimensional coordinates of the plurality of position marks.
Furthermore, the scanning unit may be constituted to emit a plurality of laser beams parallel to each other, and aligned in the auxiliary scanning direction.